1. Field of the Invention
The present invention relates to a reduction resistant thermistor able to detect temperature with a good accuracy over a broad temperature range and having characteristics stable even in a reducing atmosphere and a method of production of the same, more particularly relates to a temperature sensor for use in automobile emissions.
2. Description of the Related Art
A thermistor for use as a temperature sensor is used for measuring a broad range of temperatures from an intermediate temperature to a high temperature of about 400° C. to about 1300° C., that is, the temperature of automobile emissions, the temperature of the flame in gas-fired hot water heaters, and the temperature of heating furnaces. The characteristics of this type of thermistor are generally expressed by the resistance and resistance-temperature coefficient (temperature dependency of resistance). To handle the practical resistance of the temperature detection circuit forming part of a temperature sensor, it is required that the resistance of the thermistor be in a predetermined range (for example, a range of 100Ω to 100 kΩ in the range of the ordinary temperature of use). As a material having resistance characteristics suitable for a thermistor, a perovskite-based composite oxide material etc. is mainly used.
As a thermistor using a perovskite-based material, there is for example the one disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-325907. This discloses, as a thermistor able to be used in a broad temperature range, a thermistor exhibiting characteristics stable in a high temperature region obtained by mixing oxides of Y, Sr, Cr, Fe, Ti, etc. in a predetermined ratio and firing the mixture to obtain a complete solid solution.
Summarizing the problems to be solved by the invention, in the past, a temperature sensor for use in automobile emissions has had the sensing part, that is, the thermistor at the front end of the temperature sensor, covered by a metal case in order to prevent deposition of dust, soot, etc. from the sensed gas. When the emission becomes a high temperature of about 900° C., however, the metal case oxidizes due to the heat of the high temperature emission and the inside of the metal case easily becomes a reducing atmosphere. Therefore, there was the problem that the oxides comprising the thermistor were subjected to a reducing action and changed in resistance.
Therefore, usually, a temperature sensor is placed in an electric furnace and aged at 900 to 1000° C. for about 100 hours to stabilize the resistance. During use of a temperature sensor, however, if holes form in the metal case or the case warps etc. and therefore emission enters the case and the thermistor is exposed to a reducing atmosphere, the resistance is liable to change. Further, in recent engine control systems, the temperature sensor is mounted at a position nearer to the engine producing the high temperature emission. Therefore, the temperature sensor is exposed to the heat of higher temperature (for example, 1100 to 1200° C.) emission. With heat aging of 900 to 1000° C. for 100 hours, the metal case may reoxidize, the thermistor may be re-reduced, and the resistance may change depending on the operating mode of the engine. In this way, the problems have not been completely solved by heat aging. Further, there was the disadvantage that the number of production steps increased and therefore the temperature sensor became higher in cost.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 9-69417 discloses to form a metal case from a special metal material, for example, an alloy mainly comprised of Ni—Cr—Fe, so as to suppress changes in the atmosphere in the case and reduce the change in resistance of the thermistor. Making the metal case from a special metal material, however, results in an increase in the cost of materials and the cost of processing. Further, the problem of the change in resistance when the thermistor itself is exposed to a reducing atmosphere remains unchanged.
In this way, a thermistor which exhibits a stable resistance characteristic even under conditions where the metal case of the temperature sensor becomes a reducing atmosphere has not been obtained yet.